1 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file. See the AUTHORS file for names of contributors.
4 //
5 // BlockBuilder generates blocks where keys are prefix-compressed:
6 //
7 // When we store a key, we drop the prefix shared with the previous
8 // string. This helps reduce the space requirement significantly.
9 // Furthermore, once every K keys, we do not apply the prefix
10 // compression and store the entire key. We call this a "restart
11 // point". The tail end of the block stores the offsets of all of the
12 // restart points, and can be used to do a binary search when looking
13 // for a particular key. Values are stored as-is (without compression)
14 // immediately following the corresponding key.
15 //
16 // An entry for a particular key-value pair has the form:
17 // shared_bytes: varint32
18 // unshared_bytes: varint32
19 // value_length: varint32
20 // key_delta: char[unshared_bytes]
21 // value: char[value_length]
22 // shared_bytes == 0 for restart points.
23 //
24 // The trailer of the block has the form:
25 // restarts: uint32[num_restarts]
26 // num_restarts: uint32
27 // restarts[i] contains the offset within the block of the ith restart point.
28 29 #include "table/block_builder.h"
30 31 #include <assert.h>
32 33 #include <algorithm>
34 35 #include "leveldb/comparator.h"
36 #include "leveldb/options.h"
37 #include "util/coding.h"
38 39 namespace leveldb {
40 41 BlockBuilder::BlockBuilder(const Options* options)
42 : options_(options), restarts_(), counter_(0), finished_(false) {
43 assert(options->block_restart_interval >= 1);
44 restarts_.push_back(0); // First restart point is at offset 0
45 }
46 47 void BlockBuilder::Reset() {
48 buffer_.clear();
49 restarts_.clear();
50 restarts_.push_back(0); // First restart point is at offset 0
51 counter_ = 0;
52 finished_ = false;
53 last_key_.clear();
54 }
55 56 size_t BlockBuilder::CurrentSizeEstimate() const {
57 return (buffer_.size() + // Raw data buffer
58 restarts_.size() * sizeof(uint32_t) + // Restart array
59 sizeof(uint32_t)); // Restart array length
60 }
61 62 Slice BlockBuilder::Finish() {
63 // Append restart array
64 for (size_t i = 0; i < restarts_.size(); i++) {
65 PutFixed32(&buffer_, restarts_[i]);
66 }
67 PutFixed32(&buffer_, restarts_.size());
68 finished_ = true;
69 return Slice(buffer_);
70 }
71 72 void BlockBuilder::Add(const Slice& key, const Slice& value) {
73 Slice last_key_piece(last_key_);
74 assert(!finished_);
75 assert(counter_ <= options_->block_restart_interval);
76 assert(buffer_.empty() // No values yet?
77 || options_->comparator->Compare(key, last_key_piece) > 0);
78 size_t shared = 0;
79 if (counter_ < options_->block_restart_interval) {
80 // See how much sharing to do with previous string
81 const size_t min_length = std::min(last_key_piece.size(), key.size());
82 while ((shared < min_length) && (last_key_piece[shared] == key[shared])) {
83 shared++;
84 }
85 } else {
86 // Restart compression
87 restarts_.push_back(buffer_.size());
88 counter_ = 0;
89 }
90 const size_t non_shared = key.size() - shared;
91 92 // Add "<shared><non_shared><value_size>" to buffer_
93 PutVarint32(&buffer_, shared);
94 PutVarint32(&buffer_, non_shared);
95 PutVarint32(&buffer_, value.size());
96 97 // Add string delta to buffer_ followed by value
98 buffer_.append(key.data() + shared, non_shared);
99 buffer_.append(value.data(), value.size());
100 101 // Update state
102 last_key_.resize(shared);
103 last_key_.append(key.data() + shared, non_shared);
104 assert(Slice(last_key_) == key);
105 counter_++;
106 }
107 108 } // namespace leveldb
109